MALDI (Matrix-Assisted Laser Desorption Ionization; a laser desorption technique)-mass spectrometry of biomolecular ions was first demonstrated in parallel efforts by Tanaka et al. using small metal particles suspended in glycerol and by Karas and Hillenkamp (Int. J. Mass Spectrom. Ion Processes 1987, 78, (53)) and by Tanaka (Rapid Commun. Mass Spectrom. 88, 2, (151)) using small organic acid molecules as matrices. In using either the particle matrices or the small organic acid matrices the matrix performs the dual function of both absorbing the laser light and ionizing the non-light absorbing analyte biomolecules through specific and poorly understood chemical reactions. The particle matrices actually perform yet a third function by physisorbing the analyte from solution onto the particle surface. The organic acid matrices met with greater success in the marketplace in part due to their ease of use over wider applicable mass ranges for proteins and peptides. However, they are not completely free of defects, the most notable being the narrow band optical absorption of the excitation radiation, and the non-uniform distributed analyte during the co-crystallization of matrix and analyte.
Efforts to use the slurried small particles as matrices has languished in all but a few laboratories primarily because of the fundamental problem that the adsorbed protein must also be surrounded by just the right amount of glycerol (interestingly, while other organics have been used in place of glycerol, none appear to work nearly as well). The drying process to establish the correct amount of glycerol is dynamic under vacuum so that the “right amount” is only transitorily achieved. This leaves just a few minutes at a specific time and place near the edge of the sample droplet for acquisition of good spectra. Nevertheless the small metal particulates, because of their flat optical absorbance over a large range of wavelengths, have a huge potential advantage over organic matrices because in principle a wider variety of lasers can be used to perform the experiments. Shurenberg (Anal. Chem. 1999, 71; pp. 221–229) has reviewed the literature and performed a number of illuminating experiments, all of which establish the current understanding of these nanoparticulate matrices. In summary, for protein masses of up to around 13 kDa, the particle/glycerol system will give identical spectra as organic acid matrices (though with about an order of magnitude less sensitivity). Above this mass range the slurried particles cannot compete with the performance of chemical matrices. Any refractory particle seems to work—including carbon nanosoot and titanium nitride—as long as the particle size is significantly below 1 micron and as long as glycerol is added.
Although the MALDI technique has greatly enhanced the art of mass spectrometric analysis of biomolecules, there remains much room for improvement. It would be desirable to develop a particle based MALDI matrix that eliminates the need for glycerol addition and the concomitant problems associated with it. The ideal particle matrix would include an efficient, broadband absorber to allow one to take advantage of electromagnetic radiation sources covering a wide range of wavelengths, especially laser sources operating at wavelengths other than 337 nm from a nitrogen laser. Work by Hillenkamp and others has used pulsed infrared lasers for MALDI analysis of analytes such as peptides and oligonucleotides codeposited with water. The water acts both as matrix and proton donor and absorbs the pulsed IR laser radiation to allow time of flight mass spectrometry of the desorbed analyte. An efficient particle matrix absorber would also allow one to use low laser power excitation over a wide, nonspecific spectral range. Beyond the intrinsic wide band optical absorbance of many solids, the size of the particulates can be tailored to increase optical absorbance in certain wavelength ranges so that the matrix absorbance can be tailored to a specifically desirable excitation wavelength. The efficient particle matrix would also permit the use of smaller molar ratio of matrix/analyte, by orders of magnitude, than is possible now with small organic matrices. It would be useful to employ such a matrix in a mass spectrometric method having a chromatographic preseparation based for example on molecular shape selectivity (IMS) or based on liquid chromatography to separate isobaric matrix interference from the mass spectrum of the analyte. Finally, the ability to efficiently combine matrices with analyte to form small aerosols which can be directly ablated after introduction into a mass spectrometer has real advantages which include among others more heterogeneous distribution of analyte within the matrix and elimination of substrate effects upon the ionization process.